The Ground Based Augmentation System (GBAS) Approach Service Type D (GAST D) standard shares the mitigation of ionosphere gradient threat between the ground and airborne subsystems. This leads to the need for an ionosphere gradient monitor (IGM) located in the ground subsystem in order to ensure the integrity of the system is not compromised in the presence of an ionosphere gradient threat. Previous IGMs utilize carrier phase double difference measurements over multiple sets of ground based GPS reference receiver pairs spaced over short baselines. Due to utilizing the carrier phase data, this short baseline IGM design is limited to a maximum reference receiver separation distance (approximately 300 meters) and is limited to a maximum detectable ionosphere gradient magnitude (approximately 500 mm/km).
The short baseline IGM design is also sensitive to non-ionosphere gradients (also referred to as tropospheric gradients) which can occur frequently, and exceed the IGM detection threshold generating false monitor alarms, which cause the short baseline IGM to become ineffective at mitigating ionosphere gradients. Tropospheric gradients are a recent unexpected noise source discovered while developing the short baseline IGM. These tropospheric gradients generally occur in the afternoon on hot sunny days.
The GAST D standard also requires mitigation of errors in the satellite broadcast ephemeris data such that the probability of incorrectly detecting an ephemeris error versus the differential range error, as experienced by the aircraft, generated by this ephemeris error meets the GAST D Standards and Recommended Practices. Current approaches for mitigating the ephemeris broadcast error involve five different monitors, all of which are an indirect measurement of the differential range error being mitigated. The current five monitor design has been susceptible to false alarms which can require excluding usage of a satellite for a two day readmittance period.
For the reasons stated above and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the specification, there is a need in the art for improved systems and methods for mitigating ionosphere gradients and ephemeris errors while also reducing susceptibility to troposphere gradient noise.